No. 3] REMARKS ON NORTHERN LITHOTHAMNIA. 113 



get confluent, it often looks as if both the said kinds of repro- 

 ductive organs are developed in one and the same crust. They 

 are more distinct and frequently a little larger than the last named 

 ones, slightly more rising above the surface of the frond, indi- 

 stinctly convex or sometimes almost disc-shaped, but less distinctly 

 visible than in several other species, 200 — 300//. in diameter, with 

 a single orifice. I have seen only a few ones, which probably 

 also are by and by decorticated. 1 have not seen those of an- 

 theridia. The conceptacles do not become gradually overgrown, 

 as far as hitherto seen. The roofs frequently falls to deca^y, and 

 the scars from the conceptacles are gradually effaced by new- 

 formed tissue. 



It is questionable whether this species represents something 

 more than a northern form of Lithoph. incrustans. On the coast 

 of Norway it is well marked, but on the coasts of the British Isles 

 it seems to be difficult to draw the line, as L. incrustans f. de- 

 jpressa apparently assumes forms- which are hardly distinguishable 

 from L. orbiculatum. I have e. g. collected such a form at 

 Roundstone on the west coast of Ireland, occurring here in rock- 

 pools in the litoral region. However, L. incrustans f. depressa 

 does not at first form orbicular crusts, but always rather extended 

 and irregular ones, and the surface is less even than in L. orbi- 

 culatum. Besides, the cells seem usually to be smaller in the 

 latter than in the former species. It is, however, to be observed 

 that specimens of both species approaching each other are as yet 

 too little known and compared. The species in question also 

 approaches Lithoph. Crouani. On the other hand, specimens of 

 this species appearing on the coasts of Norway and Denmark are 

 very easily confounded with Phym. compactum, when it is sterile, 

 or when the conceptacles are not well developed, as in the latter 

 species the conceptacles of sporangia are often of about the same 

 diameter as in Lithoph. orbiculatum, the impressed part of the 

 roof, however, being intersected with a few muciferous canals, 

 though in a certain stage to a cursory view it sometimes appears 

 to be traversed by a single pore. Thus a stunted form collected 



at Fseo near Haugesund bears a striking resemblance to young 



s 



